Electrophotographic developing method utilizing a thermoplastic photo-conductive layer having entrapped solvent therein



May 30, 1967 E. c. G|A|Mo, JR 3,322,540

ELECTROPHOTOGRAPHIC DEVELOPING METHOD UTILIZING A THERMOPLASTIC PHOTOCONDUCTIVE LAYER HAVING ENTRAPPED SOLVENT THEREIN Filed April 1G, 1964 INVENTOR. E DWARD C. GIAIMo, JR.

afs. /vef Aye/f United States Patent O 3,322,540 ELECTROPHOTOGRAPHIC DEVELOPING METH- OD UTILIZING A THERMOPLASTIC PHOTO- CONDUCTIVE LAYER HAVING ENTRAPPED SOLVENT THEREIN Edward C. Giaimo, Jr., Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed Apr. 16, 1964, Ser. No. 360,212 Claims. (Cl. 961.1)

This invention relates generally to the art of electrophotography, and more particularly to improved methods of developing an electrophotographic record member, and to an electrophotog-raphic record element. The improved methods of the present invention are particularly, but not exclusively, useful for producing frosted ripple images in certain record elements.

By the term frosted, as used herein, is meant a lightscattering appearance produced primarily by the presence of small entrapped =bubbles in a transparent medium.

It has been proposed to produce a ripple image in the thermoplastic photoconductive insulating layer of an electrophotographic record element by producing an electrostatic latent image on the layer and heating the layer to allow the electrostatic forces across it to form the ripple image therein. Ripple images so obtained cannot be easily viewed directly, but they may be viewed easily with the aid lof a s-chlieren optical system. Developing methods have also been proposed to produce light-scattering portions in the ripple image of the developed record element to obviate the need for viewing the ripple image with a Schlieren optical system. Such prior art developing methods for producing these ripple images, though satisfactory for many applications, require specially prepared record elements and/ or process operations that differ from those in conventional use.

It is a general object of the present invention to provide improved methods of developing visible ima-ges in incompletely dried electrophotographic reco-rd elements of the thermoplastic photocondu-ctive insulating type.

Another object of the present invention is to provide improved methods of frosting a ripple image in an electrophotographic record element to enable one to view the ripple image without the use of a Schlieren optical system.

Still another object of the present invention is to provide improved developing methods of producing frosted ripple images in solvent containing, thermoplastic photoconductive insulating layers of record elements of the prior art.

A further object of the present invention is to provide improved developing methods of the type described that obviate the need `for applying toner particles to the record element to view an electrostatic latent image thereon.

Briefly, these and other objects and advantages of the :present invention are accomplished by improved methods of producing a frosted ripple image in an electrophotographic record element that has a thermoplastic photoconductive insulating layer with entrapped solvent therein. The frosting is caused by bubbles that are formed from the entrapped solvent in the layer when the latter is heated and softened. The time required for a bubble from each elemental quantity of solvent to be formed within the heated layer is determined by factors including, (l) the amount of heat energy absorbed by the elemental quantity of solvent, (2) the surface tension of the softened layer, and (3) the amplitude of the electrostatic charge across the layer containing the elemental quantity of solvent. The -bubbles provide a good light scattering medium, thus making a frosted ripple image, with portions containing 'bubbles therein, visible directly to the unaided eye.

In each of the methods of the present invention, a thermoplastic photoconductive insulating layer, hereinafter also called simply 1ayer, is exposed to a light image to provide a conductivity pattern therein. The layer is charged electrostatically, either before or after its exposure to the light image, and then heated to at least its softening temperature until the light-struck (electrostatically discharged) portion of the layer that has been exposed to the light image becomes frosted by bubbles, and the remaining unexposed (electrostatically charged) area of the layer becomes depressed with respect to the frosted portion. The layer may now be cooled, and the resulting If, instead of cooling the layer at this stage of the method, the layer is heated for an additional period of time at at least its softening temperature, the bubbles in the crazed portion eventually boil away and bubbles from the solvent in the depressed portion form, thereby frosting the depressed areas by bu-bbling and leaving the initially light-exposed .portions relatively smooth. The layer may now be cooled and viewed directly, or it may be heated further, if desired, until the bubbles in the depressed portions have also boiled away, leaving the depressed areas, as well as the initially light-exposed portions relatively smooth, all of the entrapped solvent in the layer having now been boiled away.

The novel features of the present invention, both as to its organization and operation, as well as additional objects and advantages thereof, will be more readily understood from the following description, when read in connection with the accompanying drawing, in which similar reference characters refer to similar parts throughout, and in which:

FIG. l is a cross-sectional view of an electrophotographic record element of the type used in the improved methods of the present invention; and

FIGS. 2, 3, 4, 5, 6, and 7 are cross-sectional views of the electrophotographic record element shown in FIG. 1, in different stages of improved methods of the present invention.

Referring, now, particularly to FIG. l of the drawing, there is shown an electrophotographic record element 10 having either a exible or a rigid substrate 12, such as Mylar or glass, for example, a transparent or reflecting conductive layer or coating 14, such as tin oxide or sprayed aluminum, lfor example, and an essentially transparent, thermoplastic photoconductive insulating layer 16. The photoconductive layer 16 may comprise a solid solution of a leucobase dye intermediate, such as the leucobase of malachite green, in a thermoplastic resin, such as styrene, for example. The solid solution comprising the layer 16 should be incompletely dried, that is, it should Contain a small amount of the solvent, such as methyl ethyl ketone and/ or toluene, for example, that was used initially to dissolve it for the purpose of applying it in a lluid form over the conductive coating 14. The layer 16 should be air-dried to cause it to harden but still to retain some of the solvent entrapped therein. When the layer 16 is heated suiciently to soften it, the entrapped solvent therein changes from the liquid state to the gaseous state, forming bubbles in the layer 16. The layer 16 should preferably be heat deformable over a relatively narrow range of temperatures.

' A crazed ripple image may be formed in the layer 16 of the record element 10 by one embodiment of the improved method of the present invention, the steps of which are illustrated in FIGS. 1, 2, 3, and 4. The layer 16 is charged electrostatically with a uniform electrostatic charge by any suitable means, as by a corona discharge device, known in the art. Referring to FIG. 2, there is shown the portion of a corona discharge device in the form of .a plurality of interconnected wires 18 from which a corona discharge may be produced. The interconnected wires 18 are connected to a voltage terminal 20, and the conductive coating 14 is connected to a voltage terminal 22, the latter being connected to a common terminal, such as ground. A source of unidirectional high voltage of about 6,00() volts (not shown) may be connected between the terminals 20 and 22 to provide the corona discharge by means of which the layer 16 may be charged electrostatically. When, for example, the negative terminal of the power supply is connected to the terminal 20, the layer 16 can be charged negatively, `as indicated in FIG. 2.

The layer 16 is now exposed to a light image to produce a conductivity pattern therein, as well as an electrostatic latent image on the layer 16. The layer 16 can be exposed to the light image through a mask 24 having dark areas 26 opaque or partially opaque to light, and light transmitting areas 28. The mask 24 may also be a photographic negative or positive, for example. Light, illustrated by the arrows 30, passing through the light-transmitting areas 28 of the mask 24 discharges the previously charged photoconductive layer 16 selectively, as at portions 29, in accordance with the intensity of the light impinging thereon, as illustrated in FIG. 3. The amplitude of the electrostatic charge across these light-struck portions 29 is either zero or relatively small. The unexposed areas, such as unexposed portions 27 of the layer 16, beneath the dark areas 26 of the mask 24, retain their electrostatic charges, producing a relatively high voltage across these areas.

If the layer 16 is now heated to at least its softening temperature, the electrostatic forces in the unexposed areas 27 produce an electrostatic pressure across these areas and cause them to become depressed, as shown in FIG. 4. The layer 16 may be heated by placing the record element on a hot plate, or, preferably, by directing a stream of hot gas, such as hot air, illustrated by the arrows 32, onto the layer 16. If, during the heating process, the electrostatic charge on the layer 16 tends to leak olf, it may be replenished by applying an additional electrostatic charge, preferably periodically, during the heating operation. Since there is relatively little or no electrostatic pressure across the light-struck portions 29 of the layer 16, the entrapped solvent in these portions of the layer 16 can pass from its liquid state to its gaseous state relatively easily. light-exposed portions 29 of the layer 16. If the developing process is terminated at this point, as by allowing the layer 16 to cool, the ripple image on the surface of the layer 16 comprises light-struck portions 29 frosted by bubbles and smooth, unexposed, depressed areas 27, as shown in FIG. 4. The ripple image may now be viewed directly in this condition with the unaided eye.

In another embodiment of the method of the present invention, results similar to those obtained in FIG. 4 may be had by producing a conductivity pattern in the layer 16, as shown in FIG. 5, by exposure to a light image, and then charging and heating the layer 16 in the manner described for the record element in FIG. 4. The bubbles of the solvent entrapped in the layer 16 should have a vapor pressure at the softening temperature of the layer 16 that is slightly less than the surface tension exerted by the softened thermoplastic layer 16 so that the bubbles remain within the light-struck portions 29 of the layer 16.

In still another embodiment of the present invention, the initially light-struck frosted portions 29 can be rendered smooth and the depressed unexposed areas 27 can be rendered frosted by bubbles, as shown in FIG. 6. To accomplish this result, the layer 16 is not cooled as soon as the light-struck portions 29 become frosted, but heat Thus, bubbles are formed rst in the 32 is applied until the bubbles that caused the frosting in the light-struck portions 29 have boiled laway and bubbles in the unexposed areas 27 have begun to form. Since the solvent entrapped in the unexposed depressed areas 27 of the layer 16 is under greater electrostatic pressure than the solvent in the exposed Iportions 29, more energy, in the form of heat, has to be absorbed by the solvent entrapped in the unexposed areas 27 to cause bubble formation and frosting. When the bubbles have disappeared from the exposed portions 29 and have appeared in the unexposed areas 27, the layer 16 is allowed to cool, freezing the bubbles in the unexposed areas 27. The ripple image on the layer 16, shown in FIG. 6, may now be viewed with the unaided eye in this condition.

Further heating of the ripple image on the layer 16 causes the bubbles to disappear, that is, to boil away, from the unexposed areas 27, and renders the unexposed depressed areas `27 smooth, as shown in FIG. 7. The ripple image on the surface of the layer 16 may now be viewed in a schliefen optical system, if so desired. Any further heating of the layer 16 will cause the layer 16 to melt and the ripple image on the layer 16 to disappear eventually.

Suitable record members 10 for use in the improved methods of the present invention may be made as follows:

Example l A coating composition A consists of 56 weight percent solvent and 44 Weight percent solids. The 56 weight percent of solvent consists of 7 weight percent methyl ethyl ketone and 49 weight percent toluene. The 44 weight percent of solids consists of 14.8 weight percent leucobase of malachite green, 27.8 weight percent polystyrene PS-2, and 1.4 weight percent chlorinated paraffin chlorowax 70. The solids are dissolved in the solvent to form the coating composition A. The coating composition A is applied to the conductive coating 14 of the record element 10 by :any well known technique, such as, for example, flow coating, dip coating, or spin coating. The coating composition A is allowed to harden by permitting most of the solvent to evaprate in air, producing the relatively thin, incompletely dried, thermoplastic photoconductive layer 16. The coating composition A is preferably incompletely dried so that a small quantity of the solvent remains in the incompletely dried layer 16. If heat is applied to the layer 16 to accelerate the hardening thereof, care should be taken not to evaporate all of the solvent from the layer because the frosting effects of the ripple images described herein are caused by bubble formation of the entrapped residual solvent. When pre-paring a transparent record element 10 with this or any other coating composition, it is preferred that a small area of the conductive coating 14 be bared of the layer 16 to provide means for electrically contacting or grounding the conductive coating 14.

Another example of a suitable coating composition for the layer 16 is as follows:

Example Il A coating composition B consists of 57 weight percent solvent and 43 weight percent solids. The 57 Weight percent of solvent consists of 27 weight percent methyl ethyl ketone and 30 weight percent toluene. The 43 weight percent of solids consists of 10.3 weight percent leucobase of malachite green, 30.9 weight percent sucrose-benzoate, and 1.8 weight percent vinylite VAGH. The coating composition B is applied to the record element 10 in the same manner as explained for the coating composition A.

Still another suitable coating composition for the layer 16 consists of the following:

Example Ill A coating composition C consists of 53 weight percent solvent and 47 weight percent solids. The 53 Weight percent of solvent consists of 21 weight percent methyl ethyl ketone and 32 Weight percent toluene. The 47 weight percent of solids consists of 13.9 weight percent leucobase of brilliant green, 32.4 weight percent sucrose-henzoate, and 0.7 weight percent vinylite VAGH. The coating composition C is applied to the record element in the same manner as explained for the coating composition A.

From the foregoing description, it will be apparent that there have been provided improved electrophotographic developing methods for producing frosted ripple images in thermoplastic photoconductive layers that contain entrapped solvent therein. While only a few embodiments of the invention have been described, variations in the operational steps, all coming within the spirit of this invention, will, no doubt, readily suggest themselves to those skilled in the art. Hence, it is desired that the foregoing shall be considered as illustrative and not in a limiting sense.

What is claimed is:

1. A method of recording on a layer of thermoplastic photoconductive insulating material, said material having a solvent entrapped therein, said method comprising the steps of (a) exposing said layer to a light image to provide a conductivity pattern therein,

(b) charging said exposed layer electrostatically to provide a charge pattern related to said light image, and

(c) heating said layer to at least the softening temperature thereof until the portion of said layer exposed to the light portions of said light image becomes frosted by bubbling and the unexposed remainder of said layer becomes depressed with respect to the frosted portion.

2. A method of recording on a layer of thermoplastic photoconductive material, said material comprising a solvent entrapped therein, said method comprising the steps of (a) exposing said layer to a light image to provide a conductivity pattern therein,

(b) charging said exposed layer electrostatically,

(c) heating said layer to at least the softening temperature thereof until the portion of said layer containing said conductivity pattern becomes frosted due to formation of bubbles of vapor of said solvent and the unexposed remainder of said layer becomes depressed with respect to said frosted portion, `and (d) further heating said layer at at least the softening temperature thereof until said frosted portion becomes smooth and said unexposed remainder becomes frosted.

3. A method of recording on a layer of thermoplastic photoconductive material, said material comprising a solvent entrapped therein, said method comprising the steps of (a) exposing said layer to a light image to provide a conductivity pattern therein,

(b) charging said exposed layer electrostatically,

(c) heating said layer to at least the softening temperature thereof until the portion of said layer corrtaining said conductivity pattern becomes frosted and the unexposed remainder of said layer becomes depressed with respect to said frosted portion,

(d) further heating said layer at at least the softening temperature thereof until said frosted portion becomes smooth and said unexposed remainder becomes frosted, and

(e) still further heating said layer at at least the softening temperature thereof until said unexposed remainder becomes smooth again.

4. A method of recording on a layer of thermoplastic photoconductive material, said layer having entrapped therein a solvent soluble in said material, said method comprising the steps of (a) electrostatically charging said layer with a uniform electrostatic charge,

(b) exposing said charged layer to a light image to provide a latent electrostatic image thereon, and

(c) heating said exposed layer to at least the softening temperature thereof until the portion of said layer exposed by said light image becomes frosted and the unexposed remaining area of said layer becomes depressed with respect to said frosted portion.

5. A method of recording on a smooth layer of thermoplastic photoconductive material, said layer having entrapped therein a solvent soluble in said material, said method comprising the steps of (a) electrostatically charging said layer with a uniform electrostatic charge,

(b) exposing said charged layer to a light image to provide a latent electrostatic image thereon,

(c) heating said exposed layer to at least the softening temperature thereof until the portion of said layer exposed by said light image becomes frosted and the unexposed remaining `area of said layer becomes depressed with respect to said frosted portion, and

(d) further heating saidlayer at at least the softening temperature thereof until said frosted portion becomes smooth and said unexposed remaining area becomes frosted.

6. A method of recording on a smooth layer of thermoplastic photoconductive material, said layer having entrapped therein a solvent soluble in said material, said method Icomprising the steps of (a) electrostatically charging said layer with a uniform electrostatic charge,

(b) exposing said charged layer to a light image to provide a latent electrostatic image thereon,

(c) heating said exposed layer to at least the softening temperature thereof until the portion `of said layer exposed by said light image becomes frosted and the unexposed remaining area of said layer becomes depressed with respect to said frosted portion,

(d) further heating said layer at at least the softening temperature thereof until said frosted portion becomes smooth and said unexposed remaining area lbecomes frosted, and

(e) further heating said layer at at least the softening temperature thereof until said unexposed remaining -area becomes smooth.

7. In a method of recording on an incompletely dried layer of thermoplastic photoconductive material, said material having a heat-extractable solvent entrapped therein, wherein an electrostatic latent image is produced on said layer, causing exposed portions of Isaid layer to be `relatively discharged and unexposed areas of said layer to have a relatively larger electrostatic charge thereacross, the improvement comprising (a) heating said layer to at least its softening temperature until said solvent in said exposed portions vapor'izes to produce a frosting effect and said unexposed areas become depressed with respect to said exposed portions, yand (-b) cooling said layer whereby to freeze a ripple image therein.

8. In a method of recording on an incompletely dried layer of thermoplastic photoconductive material, said material having a heat-extractable solvent entrapped therein, wherein an electrostatic latent image is produced on said layer, causing exposed portions of said layer to be relatively discharged and unexposed areas yof said layer to have a Irelatively larger electrostatic charge thereacross, the improvement comprising (a) heating said ilayer to at least its softening tempera- :ture until said solvent in said exposed portions goes into a vapor state to produce a bubbled effect and said unexposed charged areas become depressed With respect to said exposed portions,

(b) further heating said layer until said solvent in said exposed portions has boiled away, leaving said exposed portions smooth, and the solvent in said unexposed areas has become a vapor, producing a bubbled eect in said depressed areas, and

(c) `cooling said layer whereby to freeze a ripple image in said layer.

9. In a method of recording on a layer of thermoplastic photoconductive material, said material having Ia solvent entrapped thereon, wherein an electrostatic latent image is produced on said layer, causing exposed portions of said layer to be relatively discharged and unexposed areas of said layer to have a relatively larger electrostatic charge thereacross, the improvement comprising (a) heating said layer to at least its softening temperature until said solvent in said exposed portions begin to bubble to produce a frosting effect and said unexposed area become depressed with respect to said exposed portions,

(b) further heating said layer until the bubbles in said exposed portions boil away, leaving said exposed portions smooth, and the solvent in said unexposed areas begins to bubble, producing a frosted effect in said unexposed areas,

(c) further heating said layer until the bubbles in said unexposed areas boil away, leaving said unexposed areas smooth, and

(d) cooling said layer whereby to freeze a -ripple image therein.

10. In a method of recording on the surface of a layer of an incompletely dried thermoplastic photoconductive material, said material having a solvent entrapped therein, wherein an eletcrostatic latent image is produced on said surface of said layer, causing exposed portions of said surface to be relatively discharged and unexposed areas of said surface to have a relatively la-rger electrostatic charge thereacross, the improvement comprising,

(a) heating said layer to at least its softening temperature until said solvent in said exposed portions begins to bubble to produce a frosting effect and said un- References Cited UNITED STATES PATENTS 2,985,866 .5/1961 Norton 96-1 3,113,022 12/1963 Cassiers et al. 96-1 3,118,786 1/1964 Katchman et al 96-1 3,169,061 2/1965 Hudson 96-1.1 3,246,983 4/1966 Sus et al 96-1.6

OTHER REFERENCES Thomas: British Journal of Applied Physics, vol. 2, April 1951, ypages 98-109.

Gundlach et al.: A Cyclic Xerographic Method Based on Frost Deformation, Phot. Sci. and Eng. vol. 7, No. l, Jan-Feb. 1963 pp. 14-18.

Marco et al.: Effect of Electrostatic Fields on Free- Convection Heat Transfer from Flat Pilates, ASME Paper 63-HT-9.

NORMAN G. TORCHIN, Primary Examiner.

A. L. LIBERMAN, C. E. VAN HORN,

Assistant Examiners. 

4. A METHOD OF RECORDING ON A LAYER OF THERMOPLASTIC PHOTOCONDUCTIVE MATERIAL, SAID LAYER HAVING ENTRAPPED THEREIN A SOLVENT SOLUBLE IN SAID MATERIAL, SAID METHOD COMPRISING THE STEPS OF (A) ELECTROSTATICALLY CHARGING SAID LAYER WITH A UNIFORM ELECTROSTATIC CHARGE, (B) EXPOSING SAID CHARGES LAYER TO A LIGHT IMAGE TO PROVIDE A LATENT ELECTROSTATIC IMAGE THEREON, AND (C) HEATING SAID EXPOSED LAYER TO AT LEAST THE SOFTENING TEMPERATURE THEREOF UNTIL THE PORTION OF SAID LAYER EXPOSED BY SAID LIGHT IMAGE BECOMES FROSTED AND THE UNEXPOSED REMAINING AREA OF SAID LAYER BECOMES DEPRESSED WITH RESPECT TO SAID FROSTED PORTION. 